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## What determines the x-intercept (i.e., "zero-crossing" or "base year") of the GMSL plots?

To understand what determines the x-intercept (i.e., "base year" or "zero crossing") of the global mean sea level (GMSL) time series, we need to review how the estimation is computed. Each point in the time series plots is the area-weighted mean of all of the sea surface height anomalies (SSHA) measured by the altimeter in a single, 10-day satellite track repeat cycle. Sea surface height anomalies are the differences of the individual, altimeter-measured sea surface heights (SSH) from a modeled mean sea surface (MSS). Since we are interested in the time series of the global mean of the sea surface anomalies (i.e., how the GMSL is changing over time), the modeled mean sea surface must be referenced to a specific point in time. The modeled mean sea surface we use is the CLS01 mean sea surface (we expect to soon update this to the improved CLS11 MSS). Other MSS models exist, but their use does not greatly alter the time series of the global mean sea level or its computed rate. The CLS01 MSS we use is principally derived from the TOPEX/Poseidon altimeter data from 1993-1999 and nominally represents the mean sea surface at the center of this range in 1996. Therefore, we would expect our time series of global mean sea level (i.e, global mean sea surface height anomalies) to have a zero-crossing at around 1996. This is indeed the case for our time series, but using an improved MSS referenced to a more recent year, such as the CLS11 MSS, will shift the zero-crossing to the right in our plots. Therefore, the zero-crossing is sort of arbitrary if the desire is to analyze the time series and rate of the global mean sea level, and use of different mean sea suface models partially explains why some other research groups estimate differing time series and corresponding zero-crossings.